As is well known, image formation conducted by an electro-photographic process incorporates the process in which toner image is formed via electrostatic charge, exposure and development, the formed toner image is transferred onto a recording sheet and the toner image transferred onto the sheet is fixed.
In the fixing of this process, when toner is melted by heat and pressure to fix the image on the sheet, water evaporates from the sheet due to the heat. Further, after the fixing, the sheet is open to the outside air, and thereby absorbs water from the outside air.
Since such evaporation and absorbance of water occur at a different rate between the front and reverse surfaces of the sheet, resulting in waving or curling on the sheet, which of course is a major problem.
The waved or curled sheets cause troubles during conveyance, subsequent image processing and stacking, in the sheet processing apparatus combined to the image forming apparatus. Further, bundled sheets formed by a filing process increase the thickness due to this deformation, which results in disturbance during binding and storage. Accordingly, technologies which can decurl these sheets have been developed.
That is, in Unexamined Japanese Patent Application Publication 4-338,060 and Unexamined Japanese Patent Application Publication 5-309,971, technology is proposed in which a mechanical bending force is given to the sheet in the conveyance route, to decurl the sheet. The former further proposes the technology in which the amount of decurl can be selected, based on the type of sheet and density of the image fixed on the sheet.
Even if the decurling amount is selected to use based on the type of recording sheet, since there are various types of the sheets, it is to be understood that when only physical bending force is used, sometimes the sheet can not be decurled.
That is, relatively thick sheets are decurled by strong force, but thin sheets can not be decurled when only physical bending force is used. Specifically, bond paper for printing work can not be decurled when only physical bending force is used.